The lateral line system of fishes and amphibians provides a particularly favorable system in which to elucidate the mechanisms underlying neural evolution. The phylogenetic distribution of lateral line receptors indicates that one or more lines of neuromasts and, frequently, all electroreceptors have been lost independently in several radiations and that electroreceptors have, more rarely, re- evolved. Because phylogeny is the result of changes in the ontogenies of an ancestral population and its descendants, one must understand the development of lateral line receptors in order to understand the mechanisms underlying their evolution. A lateral line placode is the primary Ontogenetic unit that generates neuromasts and electroreceptors in salamanders and those bony fishes that have retained primitive electroreceptors, but comparable information does not exist for teleost electroreceptors that have re-evolved. Therefore, a detailed description of lateral line receptor development in the electroreceptive channel catfish will be undertaken using scanning electron microscopic, histological and immunohistochemical techniques (aim 1). Comparing the ontogenetic sequences for primitive and re- evolved receptors can establish where changes have occurred but can not establish the mechanisms underlying these changes. To determine these mechanisms, a logical first step is a better-understanding of the possible role of cephalic neural crest in the induction of lateral line placodes. This will be examined by cell lineage studies involving immunohistochemistry and embryonic transplantations in salamanders (aim 2). Finally, Hox genes, a newly discovered network of transcription factors are believed to play a major role in the patterning of neuroectodermal tissues and may also be involved in the individual specification of lateral line placodes. To test this hypothesis axolotl embryos (late gastrula or neurula stages) will be exposed to retinoic acid, which is known to produce transformations in the expression of Hox genes, and the distribution and innervation of lateral line receptors will subsequently be examined for resulting homeotic transformations (aim 3). Together, these studies will provide critical insights regarding the development of teleost electroreceptors, the mechanisms of their development, and the role of Hox genes in patterning head organization.